1. Field of the Invention
The present invention relates to a water jet propulsion watercraft including a jet unit (jet propulsion device) having an engine (internal combustion engine) as a drive source.
2. Description of the Related Art
An example of a water jet propulsion watercraft is disclosed in US 2004/0194682 A1. This water jet propulsion watercraft includes a hull, a water jet pump, and an engine. The engine applies a driving force to the water jet pump. The water jet pump includes a propeller shaft rotated by the driving force of the engine, and an impeller coupled to the propeller shaft. By rotation of the propeller shaft, water is sucked in from a hull bottom and the water is jetted rearward by the impeller. A propulsive force is thereby applied to the hull.
The engine has a crankcase at a lower side. A crankshaft housed in the crankcase is extended to a rear of the crankcase. An extended axial member is coupled to a rear end of the crankshaft. The propeller shaft is coupled to the extended axial member via a coupling.
A housing is disposed at the rear of the crankcase. The crankshaft is coupled to the extended axial member inside the housing. A power generator, which is a heavy object, is disposed inside the housing. A center of gravity of the water jet propulsion watercraft can thereby be positioned at the rear.
To dispose the power generator at the rear of the crankcase, a space arranged to dispose the power generator between the crankcase and the coupling must be secured. The extended axial member is provided between the crankshaft and the coupling to secure this space.
More specifically, a rear end portion of the crankshaft has a tapered shape portion, and further to the rear thereof, a male screw is coaxial to a rotational center axis of the crankshaft. A female screw, engageable with the male screw, is formed at a front end portion of the extended axial member.
According to the water jet propulsion watercraft, the impeller receives a load from water in a direction opposite a rotational direction of the impeller (and the crankshaft) during propulsion on water. The screws are arranged so that when the load received by the impeller is applied via the extended axial member to the engagement portions of the crankshaft and the extended axial member, the engagement portions are tightened to each other by the load. The engagement portions of the crankshaft and the extended axial member are thereby prevented from loosening during propulsion on water. Also, by the fastening of the crankshaft and the extended axial member, vibration is prevented from occurring between the crankshaft and the extended axial member.